In a conventional throttle valve system of an internal combustion engine, a throttle shaft is disposed in an air intake passage, which is formed inside of a cylindrical bore portion of a throttle housing. Moreover, a throttle valve for controlling the amount of intake air is rotatably attached to the throttle shaft. A full-close stopper is provided on an external wall of the bore portion. When an accelerator pedal is released, the throttle valve is fully closed by biasing force of a return spring. Then, an acceleration lever, which is connected with one end of the throttle valve, contacts the full close stopper. The full-close stopper includes a valve-position adjustment screw for adjusting a fully closed position of the throttle valve.
Such the throttle valve system is, for example, described in a patent document of JP-A-H10-205359. In this throttle valve system, for lightening its body, the throttle housing and the throttle valve are made of a composite material the main component of which is a synthetic resin. It is considered to employ a tapping screw fastening structure shown in FIGS. 14A and 14B. In this structure, a tapping screw 101 is used as the valve-position adjustment screw. A full-close stopper 102 to be screwed with the tapping screw 101 is formed on a bore portion of a throttle housing, which is made, for example, of a heat-resisting resin. The full close stopper 102 has a non-threaded hole 103 to be engaged with a male-threaded portion 105 of the tapping screw 101. The diameter of the non-threaded hole 103 is smaller than outer diameter of the male-threaded portion 105. Therefore, while the tapping screw 101 is screwed into the full-close stopper 102 so that the male-threaded portion 105 should be engaged with the non-threaded hole 103, very high fastening torque is applied to the tapping screw 101.
Accordingly, around the inlet of the non-threaded hole 103, some parts of the full-close stopper 102 projects as shown in FIG. 14B. Moreover, when the tapping screw 101 is further screwed into the full-close stopper portion 102, those projected parts are forcedly pressed by a screw head 104. Therefore, the full-close stopper 102 is likely to be cracked. Usually, the acceleration lever applies a high load to the tapping screw 101. Thus, when the throttle valve system is used for a long time, the full-close stopper 102 will be notable to support the tapping screw 101 stably. Consequently, valve position adjusting performance is disadvantageously lowered.